1. Field of the Invention
The present invention relates to wireless communication systems. More particular, it is related to synchronisation between the physical layers of a cellular wireless network and a wireless local area network in a heterogeneous network structure. Such synchronisation is essential for the efficient operation of a wireless communication infrastructure node, or device, that supports both of the above-mentioned systems.
2. Description of the Related Art
In packet services of cellular networks, like GSM/GPRS or UMTS, radio resources are usually administered in time intervals of pre-defined fixed length. For example in GPRS of GSM, packet channels are assigned in time units of time slots. With UMTS, data is transmitted on the physical channel in time units of transmission time intervals (TTIs). During one TTI, one so-called Transport Block Set is transferred by the physical layer on the radio interface. The TTI is always a multiple of the minimum interleaving period (e.g. 10 ms, the length of one RF (Radio Frame)). The MAC (Medium Access Control) delivers one Transport Block Set to the physical layer every TTI. Without loss of generality, UMTS terminology will be used for the description of the invention below. An extension to UMTS technology for high speed packet transmission is High Speed Downlink Packet Access (HSDPA). It allows packet-based data service in W-CDMA downlink with data transmission up to 20 Mbps. HSDPA implementations include Adaptive Modulation and Coding (AMC), Multiple-Input Multiple-Output (MIMO), Hybrid Automatic Request (HARQ), fast cell search, and advanced receiver design.
Among other applicable cellular RAN technologies there is CDMA2000 with the high speed packet transmission 1×EV-DV.
The TTI in UMTS is of fixed size; it can have values of 2, 10, 20, 40 and 80 ms. The TTI values cannot change very dynamically. Within a session, the TTI size can change from one of the above values to another value, but this involves signalling between the user terminal and the network. However, even if a change of the TTI value occurs, this implies that the TTI size changes from one value of the above-mentioned ones to another value of the group.
In addition, in UMTS there is a timing structure with numbering of frames and slots. In case UMTS cooperates tightly with another radio access technology, a radio access scheduler is needed. The radio access scheduler needs to have predefined operation intervals and a timing structure controlled by a clock or counter. For these reasons, it is fair to assume that, when UMTS is tightly cooperating with a wireless local area network the radio access scheduling period is going to be equal to a multiple M of UMTS TTIs.RadioAccessSchedulingPeriod=M×UMTS_TTI  (1)
Moreover, a radio access scheduling period is assumed to start at the beginning of a TTI.
In wireless local area networks (WLANs), like for example according to the IEEE 802.11 protocol, air interface resources have to be reserved for transmitters or transmission links in order to avoid collision of different transmissions on the same channel. As such networks generally consist of a plurality of unsynchronized devices, there is no clock to which this reservation mechanism is synchronized, and the duration of the reservation can be made variable according to the instant requirements of the transmission. Without loss of generality, IEEE 802.11g terminology will be used for the description of the invention below.
The length of a period during which the channel is reserved in IEEE 802.11g can take any value between 0 and 32767 μsec. In addition, at the end of each channel reservation, an idle period follows. This period is called inter-frame space (IFS) and it can have different values, depending on the type of coordination within the WLAN Basic Service Set (BSS). In case of Distributed Coordination Function (DCF), this inter-frame is called DIFS (DCF inter-frame space). For the case of centralized control, hence the case of Point Coordination Function (PCF), this space is called PIFS (PCF inter-frame space). These inter-frame spaces have different values because the MAC uses these spaces in order to provide implicitly multiple priorities to users. Considering that PCF control is used for (near-) real traffic, hence for traffic of higher priority than packet data, the PIFS is smaller than the DIFS. Indeed the value of PIFS is equal to 25 μsec and the value of DIFS is equal to 34 μsec. Moreover, another type of IFS is defined: it is the Extended IFS (EIFS). It is the spacing required between the station's attempt to retransmit a failed packet and any previous frame sequence. Since retransmitted packets are of lower priority than the freshly transmitted ones, the value of EIFS is higher than the value of the previous IFS; it is equal to 43 μsec.
As the usage of wireless data transmission is constantly growing, the integration of different radio access technologies into heterogeneous networks becomes more and more important. For example a traveller in a car might have access to the internet from a notebook computer via a packet oriented service of a cellular network. Upon arrival in the office, IP traffic should be seamlessly handed over to a WLAN which is connected to the Internet via land line connections, e.g. glass fibres. In a boundary area there might even occur repeated handovers between both networks. Consequently there is a need for an efficient and seamless integration of different radio access technologies into heterogeneous wireless networks.
The coupling of different radio access technologies has been the subject-matter of various patent applications.
US-2004-0185887-A1 describes a Multi-Radio Unification Protocol (MUP). Implemented in a wireless node, it coordinates the use of multiple wireless network interface cards and provides a virtual layer that hides the multiple physical network interfaces from higher layers of a node's network protocol stack.
The issue of synchronizing the operation of different radio access technologies in multi-mode wireless communication devices and infrastructure nodes is described in US-2004-0185899-A1. More precisely, the synchronization of processors in different radio access technologies (RATs) is discussed. A case is considered, in which only one of the two systems is active at a time. Therefore the case of tight coupling is not addressed. Moreover, US-2004-0185899-A1 proposes a synchronization mechanism between two timers/counters by exchanging messages and interrupt signals. It would be desirable to avoid this exchange of signals.
The idea of using different RATs for the transmission of a single traffic flow is described in US-2004-0203815-A1. The combined use of licensed and unlicensed frequency bands and consequently systems is discussed and the functions and methods for the selection of the appropriate RAT during transmission are presented. However, as this document addresses radio selection at longer time scales, it does not address the case of tight coupling.
Even if solutions described in the prior art deal with the cooperation of different radio access technologies, the problem of transmission synchronization, or time alignment, at the physical layer is not addressed. For example, in the case of tight multi-radio cooperation, at each transmission time interval (TTI), data destined for one user may be scheduled on one of the two systems. The selection may be done on the basis of radio parameters (e.g. load), service QoS parameters, (e.g. round trip time), etc, as described in US-2004-0185887-A1. For the realization of the functionality mentioned above, and of other related functions, a multi-radio unification protocol is needed. This protocol is responsible for the selection of the RAT on which the transmission is performed. Therefore, a mechanism comprising a radio access scheduler is included in MUP. In addition, MUP is responsible for the multi-RAT automatic repeat request (ARQ). An exemplary protocol stack in the case of tight cooperation between a cellular RAN and a WLAN is displayed in FIG. 2.
In the tight cooperation between a WLAN and the RAN of a cellular system, the transmission in the physical layer looks like depicted in FIG. 3. Data is transmitted on one of the two systems. The radio access scheduler operates at certain time intervals, the radio access scheduling periods.
The timings of the physical layers are independent, if no action for their synchronization is taken. In this case, the operation of the multi-RAT scheduler is usually synchronized with the operation of the RAN physical layer, as it is shown in FIG. 4.
FIG. 6a shows an example of transmission in the physical layer, when a RAN 602 and a WLAN 601 tightly cooperate. The RAN uses transmission time intervals (TTI) of fixed length 603. In addition, the procedure of data transmission in the WLAN 601 is presented according to the IEEE 802.11 standard. Data transmission is initiated by the message Request to Send (RTS) 604. Among others, this message contains a field named “Duration”. In this field, the transmitter specifies the time 605 for which it reserves the channel. This time is equal to the period needed for the data transmission plus the time required for the acknowledgement. The Duration field can have a value between 0 and 32767 μsec. If the duration value is not aligned with the TTI size in the RAN, then the end of transmission in the WLAN exceeds the boundaries of the RAN TTI and hence the WLAN is not available for the scheduling. This scenario is illustrated in FIG. 5.
In such a case, the scheduler has two options: either to allocate resources to users on the RAN, or to wait until the transmission in the WLAN ends and consequently the WLAN is free for scheduling as well. If the first option is chosen, the scheduler very likely allocates resources in contradiction to its policy. As an example, consider the case in FIG. 5 that at moment t2 user D is allocated resources in WLAN, but these resources cannot be granted to him, because the WLAN is still occupied by user A.
If the second option is chosen, time gaps 606 in the transmission may occur, as shown in FIG. 6a. 
In order to avoid time gaps in the physical layer transmission and to make feasible the operation of a multi-RAT scheduler, the two different physical layers have to be aligned in time. Consequently, there is a strong need for a mechanism in the MAC protocol of the WLAN that is providing this time alignment or synchronization.
As explained above, there is a need for time alignment of the physical layers of a cellular RAN and a WLAN in case of their tight coupling.